1. Field of the Invention
The present invention relates to a semiconductor integrated circuit and a method of controlling the same. Particularly, the present invention relates to a semiconductor integrated circuit having a differential amplifier circuit consisting of MOS transistors, and a method of controlling the same.
2. Description of the Related Art
In recent years, regulations with regard to safety have been intensified one after the other in Japan and United States. According to the TREAD Act (Transportation Recall Enhancement, Accountability and Document Act) which takes effect in North America, a new car that will be sold on 2006 or later is obligated to carry a vehicle tire pressure monitoring system. Therefore, it is currently considered to provide sensors in tires in order to measure tire pressure and temperature. More specifically, sensor units are mounted on valve sections of respective tires such that all four wheels can be monitored individually. Such a system has the advantages that the monitoring in high precision can be attained and the tire pressure can be monitored even during parking and stopping.
In the tire pressure monitoring system, the tire pressure is measured at regular time intervals, the measured data are transmitted to a body-side via radio waves, and the data are displayed on a display provided in a cockpit. Therefore, the system has a “tire-side module” provided at a tire wheel and a “body-side module” provided in the body side. The “tire-side module” has several kinds of sensors, a receiving unit and a transmission unit. The sensors are used for detecting pressure, temperature and so on. The receiving unit receives a command data which is transmitted from the above-mentioned body-side module through LF (Low Frequency) radio waves. The transmission unit transmits the data obtained by the sensors to the above-mentioned body-side module through RF (Radio Frequency) radio waves.
The command data (command signal) transmitted from the body-side module on the LF radio waves is transmitted in an ASK (Amplitude Shift Keying) format. The tire-side module receives the command signal on the LF radio waves by using an LC resonant antenna, and an input voltage resulting from LC self-excitation resonance is detected by a comparator having a high gain. Sensitivity in the detection is determined by a threshold of the comparator. The threshold is generally referred to as an “offset” of the comparator.
A typical comparator has a differential amplifier circuit. FIG. 1 shows a conventional differential amplifier circuit which has a resistance load RL. In an integrated circuit, two adjacent transistors Tr1 and Tr2 can be manufactured to have substantially the same characteristics. Therefore, influence of drift such as the temperature and the like can be eliminated in a differential amplifier circuit 60 as shown in FIG. 1. Such the differential amplifier circuit 60 is used for receiving the above-mentioned command signal in the ASK format. As shown in FIG. 1, the differential amplifier circuit 60 has a pair of bipolar transistors Tr1 and Tr2 having substantially the same characteristics. Bases of the respective transistors Tr1 and Tr2 serve as input terminals for receiving input voltages Vi1 and Vi2. Collectors of the respective transistors Tr1 and Tr2 serve as output terminals for outputting output voltages Vo1 and Vo2. Also, respective of the collectors are connected to a power supply through resistance loads RL of the same size. Emitters of the respective transistors Tr1 and Tr2 are connected to a common bias power supply. The ratio of the change in the difference between the output voltages (Vo1−Vo2) to the change in the difference between the input voltages (Vi1−Vi2) of the differential amplifier circuit 60 is referred to as a differential gain G, which is represented by the following equation:G=|(Vo1−Vo2)/(Vi1−Vi2)|=gm*RL
Here, the RL is the resistance load connected to the collectors of the transistors TR1 and TR2. The gm satisfies the following relation: gm=qIe/2kT. Here, k is a Boltzmann constant, T is an absolute temperature, and q is an elementary charge. Also, Ie is emitter currents of the pair of transistors TR1 and TR2. Thus, the differential gain G is determined in accordance with the resistance load RL and both emitter currents Ie.
In relation to the foregoing technique, Japanese Laid Open Patent application (JP-A 2004-64262) proposes a differential amplifier circuit. The differential amplifier has a pair of bipolar transistors and a pair of loads connected to outputs of the pair of transistors. The pair of loads includes a pair of capacitances, a pair of current sources and a pair of high resistances which are provided in parallel. The pair of capacitances has impedance for determining the gain of the differential amplifier circuit at a predetermined frequency. The pair of current sources is used for canceling a bias current of the differential amplifier circuit. The pair of high resistances determines output bias voltages at the output ends.
Japanese Laid Open Patent Application (JP-A-Heisei 10-68785) discloses a receiving circuit with an antenna. The receiving circuit has an analog amplifier and a filter as shown in FIG. 2. In FIG. 2, the circuit includes an input terminal 20010, an amplifier circuit 20001, an active filter circuit 20002, a crystal filter 20003, an amplifier circuit 20004, and an output terminal 20027. The amplifier circuit 20001 has a capacitance 20011, a CMOS inverter 20012 and a resistance 20013. By increasing the value of the resistance 20013, it is possible to enhance the gain of the amplifier circuit. However, noises of power supply and GND are also amplified at the same time. The amplifier shown in FIG. 2 can not be applied to a circuit sensitive to the noises.